Small volume arc discharge lamps are currently being developed for use as vehicle headlamps. These lamps offer long service lives and have a high electrical efficiency. In addition, such lamps have small optical sources and thus are well adapted to headlamp optics. The enclosed volume of such lamps is roughly that of pea, so the whole lamp may be conveniently formed initially from a small diameter quartz tube. Since the light source itself is small, the reflector optics may be made correspondingly small.
One difficulty that arises is that the lamp must be precisely located within the reflector assembly. Additionally, when ignited, the lamp becomes hot, thereby requiring that support materials for the lamp must be such that they can withstand high temperature operation. Although, because of their ability to withstand high temperatures, metals and ceramics have been used to support arc tubes in the past, these materials are hard, and precise coupling of the arc tube with a metal or ceramic support structure can cause a strain between the arc tube and the support. Since the conventional manufacture of arc tubes and support structures normally results in some variation of the parts, some manufactured lamp tubes will have high contact and stress, whereas others will have low contact and stress. The coupling stress is made worse by thermal expansion and contraction of the components of the lamp as it is turned on and off. Finally, the arc tube can be caused to fracture if a portion of it is pinched in a support structure. Thus, a need exists for an arc discharge headlamp capsule that can withstand the temperatures of operation and still allow the arc discharge tube to be precisely positioned and focussed without applying excessive stress to the tube.
A related problem concerns the energy management of the arc tube. The physics of the arc process are carefully designed for a particular operation temperature. If thermal conduction from the arc tube differs, for example where the support structure has a low or high amount of contact with the arc tube, heat can build up or be lost too quickly from the arc tube. As a result, the physics of the arc change, causing variations in lamp optics, life and color. Thus a need exists for a support structure that offers a consistent thermal contact from one arc tube to the next.
The prior art includes various attempts to overcome some of the problems described above, however, to date, these attempts have been less than ideal. Included among the prior art are the following:
U.S. Pat. No. 4,734,612 discloses a double-ended lamp capsule having leads welded to support wires. One support wire extends through an insulating sleeve and then, in parallel with the second wire, extends through a seal area of an enclosing capsule. PA1 U.S. Pat. No. 4,754,373 discloses a double-ended filamented lamp capsule positioned in an automobile headlamp reflector. The reference discloses a structure in which a double-ended capsule may be welded and aligned axially as part of the automobile headlamp. PA1 U.S. Pat. No. 5,032,758 shows an arc discharge unit axially aligned in a headlamp capsule. PA1 U.S. Pat. No. 5,036,439 discloses a double-ended arc discharge lamp capsule held in a metal reflector or shield. The reflector includes spring tabs that press against the lamp tube to properly position the shield. PA1 U.S. Pat. No. 5,039,904 discloses a double-ended arc discharge tube axially mounted in a headlamp. One end of the tube is captured in a threaded structure having O-rings. The forward lead is ducted away from the capsule base to pass through the reflector body for exterior electrical connection. PA1 U.S. Pat. No. 5,051,658 discloses a double-ended arc discharge tube having one end pinched between two metal arms. The arms are supported on a bush that is in turn connected to a tube and holder body. PA1 U.S. Pat. No. 5,059,855 discloses a double-ended arc discharge tube welded by its leads at each end. The forward lead extends back through an insulating sleeve to a base. The leads are otherwise ducted through the body of the base. PA1 U.S. Pat. No. 5,291,092 discloses a double-ended arc discharge headlamp capsule having an arc tube, wedge, retainer, forward connector rod, insulating sleeve, RF-ring, and base. The wedge is formed to mate with an end of the arc tube and to be compressed along its exterior by the retainer. The compressed wedge is said to gently, but firmly, hold the arc discharge tube.